and thermal decomposition of the diazonium tetrafluoroborate
gave 4-fluoro-o-xylene (56%) as a colourless liquid, bp 60 ЊC/20
Torr (lit.,65 143–144 ЊC/730 Torr); δH 2.11 (6 H, br s) and 6.6–7.1
(3 H, m); δC 18.8, 19.7, 112.4 (d, J 21), 116.4 (d, J 21), 130.85
(d, J 8, C-6), 132.1 (d, J 3, C-1), 138.7 (d, J 7, C-2) and 161.6
(d, J 242, C-4); m/z 124.
consisted mainly of tricyclo[6.2.0.0.3,6]deca-1,3(6),7-triene
(benzodicyclobutene) 22 (47%) [δH 3.22 (8 H, s) and 6.87 (2 H,
s); δC 29.5 (4 CH2), 117.4 (2 C) and 143.5 (4 C, 4ry); m/z 130],
with a trace amount of 1,2,4,5-tetramethylbenzene as the only
byproduct.
Photochemical bromination of this product with bromine in
CCl4 gave 17 (77%) as a colourless lachrymatory liquid which
solidified with time, mp 61 ЊC (lit.,23 60 ЊC); δH 4.56 (2 H, s), 4.58
(2 H, s), 6.97 (1 H, m), 7.09 (1 H, m) and 7.32 (1 H, m); δC 29.0,
29.2, 116.2 (d, J 21), 117.8 (d, J 23), 132.3 (d, J 3, C-1), 132.8
(d, J 9, C-6), 138.8 (d, J 8, C-2) and 162.4 (d, J 250, C-4);
m/z 280/282/284 (1 : 2 : 1).
ꢀ,ꢀЈ-Dichloro-p-xylene 23. FVP of the title compound
(1.05 g, 1.2 g magnesium) gave a yellow polymer which was
insoluble in conventional solvents. Microanalysis of the
material gave a result indicative of poly(α-chloro-p-xylylene) 25
(Found: C, 66.9; H, 5.9. (C8H7Cl)n requires C, 69.3; H, 5.1%).
Assuming this formula, the yield of the polymer was 61%.
Thermal analysis on the material showed that it was stable
in a nitrogen atmosphere up to 360 ЊC and in air up to 300 ЊC,
after which there was a gradual weight loss up to 550–600 ЊC
at which point it decomposed. The degradation point in
nitrogen was sharp while in air it was gradual.
Repeat pyrolysis of the title compound using a higher
magnesium to substrate ratio (0.72 g, 2.0 g magnesium) gave a
white brittle polymer, which was also insoluble in conventional
solvents. Microanalysis of this material gave a result indicative
of poly(p-xylylene) 27 (Found: C, 90.0; H, 7.8. (C8H8)n requires
C, 92.3; H, 7.7%). Assuming this formula, the yield of the
polymer was 72%. Solid state 13C NMR of the material gave
results consistent with those expected for poly(p-xylylene);
δC 39.9 (CH2), 129.2 (CH), and 139.7 (4ry). Using a small CH3
(δC 21.1) end-group signal, the polymer was estimated to consist
of 21 units, giving an approximate molecular weight of 2184.
Thermogravimetric analysis showed that the polymer was
stable up to 520 ЊC in nitrogen and 480 ЊC in air. There was no
weight loss in nitrogen up to 520 ЊC while in air a steady loss of
about 40% weight was observed over the range 300–500 ЊC.
ꢀ,ꢀЈ-Dibromo-3-fluoro-o-xylene 19. Diazotisation of 3-amino-
o-xylene with NaNO2–HCl followed by reaction with NaBF4
and thermal decomposition of the diazonium tetrafluoroborate
gave 3-fluoro-o-xylene (47%) as a colourless liquid, bp 60 ЊC/20
Torr (lit.,23 58.5–60 ЊC/26 Torr); δH 2.10 (3 H, d, J 2), 2.18 (3 H,
br s) and 6.8–7.1 (3 H, m); δC 10.6 (d, J 6, 2-Me), 19.5 (d, J 3,
1-Me), 112.7 (d, J 23, C-4), 123.6 (d, J 16, C-2), 125.3 (d, J 2,
C-6), 126.5 (d, J 9, C-5), 139.2 (d, J 4, C-1) and 161.7 (d, J 243,
C-3); m/z 124.
Photochemical bromination of this product with bromine in
CCl4 gave 19 (84%) as a colourless lachrymatory liquid which
solidified with time, mp 41–42 ЊC (lit.,66 41–42.5 ЊC); δH 4.73
(2 H, s), 4.81 (2 H, d, J 2) and 7.0–7.5 (3 H, m); δC 21.0 (d, J 7,
2-CH2), 28.9 (d, J 3, 1-CH2), 116.2 (d, J 22, C-4), 125.0 (d, J 15,
C-2), 127.1 (d, J 3, C-6), 131.1 (d, J 10, C-5), 139.3 (C-1) and
161.7 (d, J 250, C-3); m/z 280/282/284 (1 : 2 : 1).
ꢀ,ꢀЈ,2,3,5,6-Hexabromo-p-xylene 28. Photochemical brom-
ination of 2,3,5,6-tetrabromo-p-xylene using bromine in CCl4
gave 28 (98%) as colourless crystals insoluble in conventional
solvents, mp 271 ЊC (lit.,26 268–269 ЊC) (Found: C, 16.8; H, 0.6.
C8H4Br6 requires C, 16.6; H, 0.7%); δC (solid state) 42.0 (2 CH2),
125.0 (4 C) and 145.0 (2 C); m/z 574/576/578/580/582/584/586
(Mϩ, 0.5/3/8/10/8/3/0.5%), 498/500 (82/83), 419 (100), 338 (15),
259 (18), 179 (5) and 98 (8).
ꢀ,ꢀЈ,2,3,5,6-Hexabromo-p-xylene 28. FVP of the title com-
pound (0.67 g) gave nothing in the cold-trap. However there
was a polymeric material and solid in the inlet tube. Micro-
analysis and the melting point of the solid, indicated that it was
just recovered starting material (0.32 g, 48%). Microanalysis of
the polymer indicated that it did not correspond to the expected
compositions (Found: C, 17.3; H, 1.7%. (C8H3Br5)n requires C,
19.3; H, 0.6% and (C8H4Br4)n requires C, 22.9; H, 1.0%).
Repeat pyrolysis at 650 ЊC without magnesium gave a white
solid at the furnace exit with red elemental bromine in the cold
trap. Microanalysis of the solid gave a result in good agreement
with expectation for poly(α,2,3,5,6-pentabromo-p-xylylene) 29
(Found: C, 19.4; H, 1.0. (C8H3Br5)n requires C, 19.3%; H, 0.6%).
Assuming this composition the yield was 69%.
ꢀ,ꢀЈ-Dichloro-o-xylene 16. FVP of the title compound (0.6 g)
gave a liquid as the major product, with a small amount of
semi-solid. 1H NMR and GCMS analysis of the liquid showed
that it consisted of benzocyclobutene (73%), o-xylene (15%)
and a trace of toluene. The small amount of semi-solid con-
sisted of a complex mixture of dimeric products including 2,2Ј-
dimethylbibenzyl, 2,2Ј-dimethylstilbene and 5,6,11,12-tetra-
hydrodibenzo[a,e]cyclooctene (total yield <5%).
Pyrolysis of other haloalkylbenzenes over magnesium
ꢀ,ꢀЈ-Dibromo-4-fluoro-o-xylene 17. FVP of the title com-
pound (1.27 g) gave a colourless liquid which consisted of
4-fluorobenzocyclobutene (80%) and 4-fluoro-o-xylene (8%).
Kugelrohr distillation gave pure 4-fluorobenzocyclobutene 18,
bp 60 ЊC/20 Torr (lit.,67 153–154 ЊC/760 Torr); δH 2.95 (4 H, s),
6.68 (1 H, m) and 6.75–6.9 (2 H, m); δC 28.9 (1-CH2), 29.0 (d,
J 2, 2-CH2), 110.5 (d, J 22), 114.2 (d, J 23), 124.1 (d, J 9, C-6),
140.8 (d, J 3, C-1), 147.0 (d, J 7, C-2) and 163.0 (d, J 243, C-4).
The required substrates were commercially available and were
used without further purification except for 1-bromo-2-phenyl-
propane, 1,1-diphenoxyethane and β-chloroacrylophenone
which were prepared as follows.
1-Bromo-2-phenylpropane 34. To 2-phenylpropan-1-ol (6.3 g,
46.3 mmol) cooled in an ice-bath, phosphorus tribromide
(4.2 g, 15.5 mmol) was added dropwise and the mixture stirred
for 18 h. It was then heated on a waterbath for 1 h, cooled,
poured into ice–water and extracted with ether. The organic
layer was dried, evaporated and the residue Kugelrohr distilled
to give 34 (6.4 g, 70%) as a colourless liquid, bp 100 ЊC/20 Torr
(lit.,68 106–108 ЊC/18 Torr); δH 1.35 (3 H, d, J 7), 3.0–3.6 (3 H,
m) and 7.30 (5 H, m).
ꢀ,ꢀЈ-Dibromo-3-fluoro-o-xylene 19. FVP of the title com-
pound (1.04 g) gave a liquid product, which consisted of 3-
fluorobenzocyclobutene (77%) and 3-fluoro-o-xylene (8%).
Kugelrohr distillation gave pure 3-fluorobenzocyclobutene 20,
bp 60 ЊC/20 Torr (lit.,23 74 ЊC/100 Torr); δH 3.00 (4 H, br s), 6.75
(2 H, m) and 7.05 (1 H, m); δC 27.1, 30.0, 113.7 (d, J 21, C-4),
119.1 (d, J 4, C-6), 129.2 (d, J 6, C-5), 129.7 (d, J 16, C-2), 148.8
(d, J 10, C-1) and 156.3 (d, J 255, C-3).
1,1-Diphenoxyethane 39. 2-Bromoethyl phenyl ether 37 was
first dehydrobrominated by heating under reflux with KOH69
to give phenyl vinyl ether 38, bp 154–155 ЊC (lit.,70 155–156 ЊC);
δH 4.45 (1 H, dd, J 6, 2), 4.82 (1 H, dd, J 14, 2), 6.70 (1H, dd,
J 14, 6) and 7.0–7.5 (5 H, m). This was then reacted with phenol
1,2,4,5-Tetrakis(bromomethyl)benzene 21. FVP of the title
compound (0.57 g) gave a white solid in the cold trap which
J. Chem. Soc., Perkin Trans. 1, 2002, 402–415
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